An optical disk that has been known as a CD or a DVD is supported by consumers as a random access information recording medium that is high in capacity and low in capacity, and widely spread. In recent years, a personal computer or an audio visual device has been sophisticated, and the amount of information that is dealt with by a user is remarkably increased. Under the above circumstances, the optical disk is demanded to provide higher capacity as the high capacity information recording medium.
Recording of information in the optical disk is conducted by irradiating a laser beam that has been modulated on the basis of record data on an information recording layer of the optical disk medium and then changing a state of the information recording layer due to a generated heat. On the other hand, the production of information is conducted by irradiating the laser beam on the information recording layer and then detecting a change in the reflectivity which is attributable to a change in the state of the information recording layer.
In the optical disk, information is recorded by changing the state of the information recording layer due to a heat that is produced by an optical spot formed by converging the laser beam on the recording medium. The information is reproduced by reading the change in the reflectivity which is caused by the change in the state with the optical spot. For that reason, the information recording density of the optical disk can be improved by lessening the size of the optical spot, thereby making it possible to increase the recording capacity per one disk. There has been known that the diameter of the optical spot which is formed on the information recording layer is proportional to λ/NA when it I assumed that the wavelength of the laser beam is λ, and the numerical aperture of the objective lens is NA. For that reason, shortening wavelength of the laser beam and increasing numerical aperture of the objective lens are effective to improve in the information recording density of the optical disk.
In recent years, there has been developed a high-density optical disk having a recording capacity of about 25 GB per information recording layer by a technique in which the optical spot is lessened by using a blue semiconductor laser of λ=405 nm and an objective lens of NA=0.85 instead of a red semiconductor laser of λ=650 nm and an objective lens of NA=0.6 used in the DVD (recording capacity=4.7 GB per information recording layer).
However, an increase in the NA of the objective lens in order to increase the recording density leads to a problem of a distortion of the optical spot due to the spherical aberration. Because the spherical aberration is approximately proportional to NA4, the optical disk system using the objective lens of the high NA generates a large spherical aberration with respect to a slight error of a cover layer thickness, to thereby remarkably deteriorate the quality of a readout signal.
As a standard that has been frequently employed, it is necessary to suppress an RMS wave front aberration within 0.07 λrms in order to reproduce the information without any error. According to this standard, it is necessary to suppress an error in the cover layer thickness of the disk within about ±4 μm. For that reason, it is necessary to control the thickness of the cover layer with a high precision in the disk manufacturing process, and there arises such a problem that the manufacturing costs cannot be suppressed.
For that reason, in the optical disk system using the objective lens of the high NA, it not enough to adjust the amount of focus offset in order to adjust the focus position to the information recording layer. Also, it is essential to provide a means for reducing the spherical aberration according to the error in the cover layer thickness in each of the disks.
As a technique for reducing the spherical aberration, there has been used a technique in which a spherical aberration correcting means is disposed in an optical path of the laser beam, the spherical aberration of a predetermined amount is added to the optical flux that passes through the spherical aberration correcting means in advance with the result that the spherical aberration generated by an error of the disk cover layer thickness when the laser beam is converged on the recording layer by the objective lens and the spherical aberration that has been added by the spherical aberration connecting means in advance cancel each other. As the spherical aberration correcting means that is used in this example, there has been known (1) a spherical aberration correcting means that is made up of two groups of objective lenses and changes a distance between the respective objective lenses by an actuator, and (2) a spherical aberration correcting means that is made up of a liquid crystal element that has been divided into plural regions and controls the refractive indexes of the respective regions by applying a voltage. For example, those spherical aberration correcting means are disclosed in, for example, JP-A 11388/2000 and JP-A 222838/2001.
There are various known examples as an index for determining a spherical aberration amount to be added in advance. For example, in a technique that is disclosed in JP-A No. 11388/2000, the adjustment of the focus offset amount and the spherical aberration correction amount is implemented by a change in the amplitude of a readout signal (RF signal) obtained when reproducing data that has been recorded in an optical disk. However, there has arisen such a problem that the adjustment of the focus offset amount and the spherical aberration correction amount could not be implemented with respect to an unrecorded disk in which no data is recorded (there is no recording mark) because the readout signal is not obtained even if the amount of focus offset and the amount of spherical aberration correction are going to be optimized.
Also, in a technique that is disclosed in JP-A No. 222838/2001, it is necessary to record any data on an information recording layer of the optical disk because the adjustment of the amount of focus offset and the amount of spherical aberration correction is implemented by using the amplitude of the readout signal of data, or both of the amplitude of the readout signal of the data and the amplitude of the tracking error signal. In other words, necessary readout signal cannot be obtained in the optical disk on which data is not recorded at all, resulting in such a problem that the adjustment of the spherical aberration correction amount cannot be implemented.
As techniques for solving the above problem, there have been known techniques by which the amount of focus offset and the amount of spherical aberration correction are adjusted on the basis of the tracking error signal or the wobble signal which can be acquired even from the unrecorded disk. Those techniques are disclosed in, for example, JP-A No. 233917/2003, JP-A No. 168225/2003, JP-A No. 171630/2004, JP-A No. 241081/2004, and JP-A No. 241102/2004.
The tracking error signal is produced by the positional replacement of the light spot from the center line of the track, and detected as a differential signal of the respective outputs of photodetectors that have been divided into two in the radius direction of the disk. The tracking error signal becomes zero (0) in the case where the. center of the light spot is positioned on the central line of the track, and takes the maximum absolute value in the case where the center of the light spot is positioned at an end of the track. The amplitude of the tracking error signal is a difference between the maximum and the minimum of the tracking error signals.
Also, the wobble means the fine slide of the track in the radius direction, and in general, the frequency, the phase, or the amplitude is modulated on the basis of data such as address information for specifying the position of the track on the disk. A method of detecting the wobble signal is basically identical with that of the tracking error signal. In the case where the repetition frequency of the wobble is sufficiently larger than the frequency band of the tracking, the light spot cannot follow the wobble, and the slide is detected as the tracking error signal.